Vacuum pumping system

ABSTRACT

The present invention relates to a vacuum pumping system ( 10 ) for evacuating a vacuum chamber ( 12 ), the system comprising: a vacuum pump ( 16 ); and a plurality of forelines ( 22, 24 ) for conveying gas to the vacuum pump wherein in a first low vacuum stage of chamber evacuation a first foreline arrangement ( 22 ) can be connected for conveying gas to the vacuum pump and in a second higher vacuum stage of chamber evacuation a second foreline arrangement ( 22, 24 ) comprising one or more of said forelines can be connected for conveying gas to the vacuum pump, wherein the second foreline arrangement has a total cross-sectional area for conveying fluid which is larger than a total cross-sectional area of the first foreline arrangement.

The present invention relates to a vacuum pumping system for evacuating a vacuum chamber.

The manufacture of many articles requires the use of a vacuum chamber. For example, processing of silicon wafers takes place in at high vacuum. Additionally, such apparatus as flat panel displays and solar cells require processing in vacuum chambers. In these latter examples, the vacuum chamber may be required to have a relatively large volume for processing relatively large articles. Typically, as part of a processing cycle, the pressure in a vacuum chamber is required to cycle between atmosphere (1 bar) and a processing pressure (e.g. 0.01 mbar). In order to improve manufacturing speed and efficiency, it is desirable to increase the rate at which gas can be evacuated from a vacuum chamber by a vacuum pumping system.

A vacuum pumping system may comprise a vacuum pump and a foreline connecting an inlet of the vacuum pump to a vacuum chamber so that the pump can evacuate gas from the chamber. The rate at which gas can be evacuated depends on for example the compression and capacity of the pump and also conductance of the foreline. It is therefore desirable to provide a foreline with a large conductance so that it provides relatively little resistance to evacuation of the vacuum chamber. If foreline conductance is small the rate at which pressure can be reduced, particularly at low pressures, can become very slow. Moreover, a high vacuum target pressure may be unattainable if the conductance is too low. A large conductance may be provided using a pipe with a large cross-section or diameter. However, a large cross-section increases the internal volume of the foreline, and if the vacuum pump is spaced some distance away from the chamber, for example in a basement pumping system, the internal volume of the foreline may become relatively large. If the volume of the chamber is comparable with the volume of the foreline, pump-down time may be adversely affected due to the large overall volume of the vacuum chamber and the foreline. In this regard, it will be appreciated that the volume of the foreline requires evacuation to the desired pressure in addition to evacuation of the vacuum chamber.

Hereto, the rate at which a vacuum chamber can be evacuated has been increased by providing a vacuum pump or pumps with greater pumping speed or compression. However, such pumps are generally larger and consume more power.

It is an object of the present invention to provide an improved vacuum pumping system.

The present invention provides a vacuum pumping system for evacuating a vacuum chamber, the system comprising: a vacuum pump; and a plurality of forelines for conveying gas to the vacuum pump from said vacuum chamber wherein in a first low vacuum stage of chamber evacuation a first foreline arrangement can be connected for conveying gas to the vacuum pump and in a second higher vacuum, i.e. lower pressure, stage of chamber evacuation a second foreline arrangement comprising one or more of said forelines can be connected for conveying gas to the vacuum pump, wherein the second foreline arrangement has a total cross-sectional area for conveying fluid which is larger than a total cross-sectional area of the first foreline arrangement.

The present invention also provides a method of evacuating a vacuum chamber, the method comprising: connecting a first foreline arrangement for conveying fluid to a vacuum pump and evacuating gas from said vacuum chamber through the first foreline arrangement in a first relatively low vacuum stage of chamber evacuation; and connecting a second foreline arrangement for conveying gas to the vacuum pump and evacuating gas through the second foreline arrangement in a second higher vacuum stage of chamber evacuation, wherein the second foreline arrangement is configured to have a total cross-sectional area which is larger than the total cross-sectional area of the first foreline arrangement.

In a third aspect the present invention provides a vacuum chamber evacuation apparatus comprising a plurality of foreline arrangements for connecting a vacuum pump to a vacuum chamber for evacuation thereof; the foreline arrangements comprising a plurality of conduits and valves configurable to form a first and a second foreline arrangement between a vacuum chamber and a vacuum pump; said second foreline arrangement being of a higher conductance than said first foreline arrangement; and wherein said apparatus is configured, in use, to switch from the first arrangement to the second arrangement when the pressure in the vacuum chamber drops below a threshold pressure.

In order that the present invention may be well understood, two embodiments thereof, which are given by way of example only, will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a first vacuum pumping system and vacuum chamber;

FIG. 2 shows a graph plotting chamber pressure against elapsed time for four vacuum pumping systems; and

FIG. 3 is a schematic diagram of a second vacuum pumping system and vacuum chamber.

Referring to FIG. 1, a vacuum pumping system 10 is shown for evacuating a vacuum chamber 12. The vacuum pumping system comprises a vacuum pump 16, such as a roots, claw or scroll pump, for evacuating the vacuum chamber 12 to between around 1 mbar to 0.01 mbar. Two or more such pumps may be provided in series or parallel and the term vacuum pump is to be construed accordingly. A plurality of forelines 20, or conduits, connects the vacuum pump 16 to the vacuum chamber 12 for conveying fluid from the chamber to the vacuum pump. A first foreline arrangement comprises one or more of the forelines and has a first total cross-sectional area for conveying fluid. The first foreline arrangement can be connected for conveying gas to the vacuum pump during a low vacuum stage of chamber evacuation. A second foreline arrangement comprises one or more forelines and has a second total cross-sectional area. The second foreline arrangement can be connected for conveying gas to the vacuum pump during a higher vacuum stage of chamber evacuation.

The total cross-sectional area of the first foreline arrangement is sized appropriately for conveying fluid at a low vacuum. Typically, at low vacuum, the foreline conductance is high and therefore a smaller cross-sectional area is adequate to prevent restriction of the pumping speed. Further, a relatively small total cross-sectional area of the first foreline arrangement reduces the total volume of the vacuum chamber and the foreline arrangement. The total cross-sectional area of the second foreline arrangement is sized appropriately for conveying fluid at a higher vacuum. The total cross-sectional area of the second foreline arrangement is sized so that speed of evacuation from the vacuum chamber is limited by a pumping speed of the vacuum pump and not by the conductance of the second foreline arrangement. Therefore, the rate of chamber evacuation can be increased and higher vacuum pressures be obtained.

In the example shown in FIG. 1, there are two forelines 22, 24. The vacuum pumping system comprises valves 26, 28 30. Main valve 26 is operable for connecting the vacuum chamber 12 to the vacuum pumping system 10 along the first foreline 22 or the second foreline 24. Valves 28 and 30 are upstream and downstream respectively of the second foreline 24. In the first foreline arrangement, valves 28 and 30 are closed to isolate the second foreline 24 from the first foreline valve 22 and therefore when main valve 26 is open fluid is conveyed to the pump along foreline 22 only. In the second foreline arrangement, valves 28, 30 are opened so that fluid is conveyed to the vacuum pump along both the first foreline 22 and the second foreline 24.

As shown, the first foreline 22 has a cross-sectional area which is less than that of the second foreline 24. The first foreline may have a cross-sectional area in the range of 10⁴ to 10⁵ mm² and the second foreline may have a cross-sectional area in the range of 10⁵ to 10⁶ mm². For example, the forelines may be circular in cross-section and the first foreline may have a diameter of 100 mm and a cross-sectional area of around 8,000 mm² and the second foreline may have a diameter of 320 mm and a cross-sectional area of around 80,000 mm². The lengths of the forelines 22, 24 are approximately the same and therefore the first foreline has a conductance and a volume which is less than the conductance and volume of the second foreline. Accordingly, in the second foreline arrangement, the second foreline only may be connected for conveying gas to the vacuum pump. However, it is preferable that foreline 22 is used in addition to foreline 24 so that gas can be conveyed through both forelines to increase the total cross-sectional area and the conductance of the second foreline arrangement.

Alternatively, two forelines of comparable size can be adopted such that in the first foreline arrangement a single foreline conveys gas to the vacuum pump and in the second foreline arrangement both forelines convey gas to the vacuum pump.

Still further, many forelines may be provided and a first plurality of forelines can be selected for conveying gas to the vacuum pump in the first foreline arrangement and a second plurality of forelines can be selected for conveying gas to the vacuum pump in the second foreline arrangement. The second foreline arrangement may comprise one or more of the forelines of the first foreline arrangement.

In many vacuum pumping systems, the vacuum pump may be spaced some distance away from the vacuum chamber, for example in a semiconductor fabrication plant where the vacuum pump may be located in a basement and connected to a vacuum chamber which is located in a clean room on a floor above. It may be necessary that the paths of the forelines turn a number of times in order to connect the pump to the chamber. Each turn, and the angle through which the forelines turn, affects conductance since a large number of turns decreases conductance. Therefore, the distance and number of turns are taken into account when determining conductance of the first and second foreline arrangements.

Reference will now be made to the graph shown in FIG. 2. The graph plots chamber pressure in mbar on the y-axis against elapsed time from commencement of chamber evacuation in seconds on the x-axis.

The graph shows curves for 160 mm, 250 mm, 320 mm and 320/100 mm forelines, the last of which is an example of the system shown in FIG. 1. The vacuum chamber tested is 1 m³ and each of the forelines is 15 m in length having 5 bends between the vacuum chamber and the pump.

For a single 160 mm diameter foreline, the plot is initially relatively steep showing that a relatively low conductance and low volume foreline allows an increased rate of chamber evacuation during a low vacuum stage. In this regard, there is less volume in a 160 mm foreline and therefore the total volume of the chamber and foreline that is required to be evacuated is reduced. However, during a higher vacuum stage, the plot shallows as the relatively low conductance of the 160 mm foreline restricts the amount of gas that can be evacuated from the chamber. In this example, the limited conductance prevents the vacuum pumping system from evacuating the chamber to the target pressure of 0.01 mbar. Instead the plot plateaus at around 0.03 mbar.

For a single 250 mm diameter foreline, the plot is initially shallower compared to the 160 mm foreline because the 250 mm foreline has greater volume to evacuate. However, as the conductance of a 250 mm is greater than that of a 160 mm foreline, the vacuum pumping system comprising a 250 mm is capable of obtaining the target pressure of 0.01 mbar after 60 seconds.

For a single 320 mm foreline, the plot is again shallower compared to the 250 mm foreline because the 320 mm foreline has greater volume to evacuate. However, as conductance of a 320 mm foreline is greater than that of a 250 mm foreline, a vacuum pumping system comprising a 320 mm foreline is capable of obtaining the target pressure of 0.01 mbar after only 48 seconds.

For the two stage 320/100 mm diameter forelines in accordance with FIG. 1, on commencement of pump down at time 0, the valve 26 is operated to convey gas to the vacuum pump along the 100 mm foreline 22 only. Valves 28, 30 are closed to isolate the 320 mm foreline 24. Accordingly, in the first foreline arrangement during a low vacuum stage of chamber evacuation, gas is evacuated relatively quickly from the chamber through the relatively small cross-sectional area 100 mm foreline 22. When the chamber reaches a predetermined pressure, which in this example is 1 mbar, or after a predetermined elapsed time, which in this example is 24 seconds, the system switches from the first foreline arrangement to the second foreline arrangement. In more detail, valves 28, 30 are opened to allow gas along foreline 24 and thus gas is conveyed from the chamber (12) to the pump (10) along both the 100 mm foreline 22 and the 320 mm foreline 24. Accordingly, in the second foreline arrangement during a higher vacuum, i.e. lower pressure, stage of chamber evacuation, gas is still evacuated relatively quickly from the chamber through the high total conductance forelines 22, 24. Therefore, the target pressure of 0.01 mbar is reached after only 42 seconds. Given that in some circumstances, processing in vacuum chamber may take place over many cycles for much, if not all, of a 24 hour period, the vacuum pumping system 10 allows a substantially overall time saving.

After pump evacuation is complete, and processing has occurred, the vacuum chamber is allowed to return to atmosphere so that processed articles can be removed. Prior to allowing the vacuum chamber to return to atmosphere, valves 28, 30 are closed to isolate the foreline 24 from the rest of the system. Accordingly, foreline 24 remains at a higher vacuum. It is preferable that the 320 mm foreline (24) has already been evacuated to a desired low pressure at the start of the chamber pumpdown. If the 320 mm foreline isn't already evacuated, the initial pumpdown will be slightly slower than a standard pumpdown, but subsequent pumpdowns will be faster. During subsequent pump downs of the vacuum chamber, when the system switches from the first foreline arrangement to the second foreline arrangement, a pressure gradient is generated between the vacuum chamber at a pressure in the region of 1 mbar and the pre evacuated foreline 24 which is at a pressure in the region of 0.01 mbar. Equalisation of the pressure causes a rapid reduction in chamber pressure as can be seen from FIG. 2, in which the plot is almost vertical between a pressure of 1 mbar and 0.35 mbar after 24 seconds of elapsed time.

Referring again to FIG. 1, a control unit 32 is connected by control lines (shown in broken lines) to the main isolation valve 26, the upstream secondary valve 28 and the downstream secondary valve 30. The control unit 32 may be a suitable programmed computer or bespoke processing unit. The control unit is configured to control the valves in order to select the first foreline arrangement or the second foreline arrangement according to a pressure in the vacuum chamber or elapsed time. If the chamber pressure has a pressure gauge capable of outputting a pressure signal to the control unit then such a pressure gauge may be used and does not form part of the vacuum pumping system 10. Alternatively, the vacuum pumping system 10 may comprises a pressure gauge or sensor 34 for sensing a pressure and outputting a pressure signal to the control unit 32. The pressure can be measured for example in foreline 22. In the absence of a pressure sensor, a clock circuit may be provided for outputting a signal to the processing unit after a predetermined elapsed time so that the processing unit can switch between the first foreline arrangement and the second foreline arrangement. For example, the elapsed time may be between 20 and 30 seconds or 24 seconds as shown in FIG. 2.

In the example shown in the graph, the predetermined pressure at which the system changes from the first to the second foreline arrangement is 1 mbar although may be in the range of 0.1 mbar to 10 mbar. Preferably, the system switches at a point where the gradient of the first low conductance foreline arrangement begins to shallow and chamber evacuation slows.

A second vacuum pumping system 40 is shown in FIG. 3 and is adapted for evacuating a vacuum chamber to a pressure in the region of 10⁻³ to 10⁻⁷ mbar. The second vacuum system comprises a vacuum pump 14, such as a turbomolecular pump, connected for conveying fluid from the vacuum chamber through at least one of the foreline arrangements to a vacuum pump 16 as described with reference to FIG. 1 above. In this example, vacuum pump 16 serves as a backing pump for the turbomolecular pump 14. As a turbomolecular pump cannot be operated at low vacuum without causing damage to the pump or causing the pump to stall, vacuum pump 16 is initially operated to lower the pressure in the chamber and the turbomolecular pump to a pressure at which the turbomolecular pump can be operated safely. A suitable safe pressure may be in the region of 1 mbar to 10⁻³ mbar. In order to achieve a reduction in pressure to a suitable safe pressure vacuum pump 16 is connected to vacuum pump 14 with forelines similar to those described with reference to FIG. 1 and for brevity the full description of the FIGS. 1 and 2 arrangement with respect to the forelines will not repeated. Briefly though, in FIG. 3, the vacuum pump 14 is connected to the vacuum chamber through a main valve 18 which can be closed to isolate the vacuum pump 14 from the chamber and opened to allow gas to flow to the vacuum pump 14. A line 42 connects the exhaust of vacuum pump 14 to the foreline 24.

A plurality of forelines 20 connects the vacuum pump 16 to chamber 12 for conveying gas evacuated from the chamber 12 to the vacuum pump 16. A first foreline arrangement comprises one or more of the forelines and has a first total cross-sectional area. The first foreline arrangement can be connected for conveying gas to the vacuum pump 16 during a low vacuum stage of chamber evacuation (e.g. to a pressure of 1 mbar). That is, chamber 12 is initially evacuated through foreline 22. A second foreline arrangement comprises one or more forelines and has a second total cross-sectional area which is larger than the first cross-sectional area. The second foreline arrangement can be connected for conveying gas to the vacuum pump 16 during a higher vacuum stage of chamber evacuation (e.g. below a pressure of 1 mbar). That is, chamber 12 is evacuated through both forelines 22, 24 during a second stage of evacuation. In this example, it is not safe to operate vacuum pump 14 above 1 mbar and therefore, vacuum line 42 need be connected only to foreline 24 since the vacuum pumping system switches to the second foreline arrangement at a pressure above the safe operating pressure of pump 14. If the safe operating pressure of the vacuum pump 14 were above the switch pressure (e.g. above 1 mbar) the vacuum line could be connected to both forelines 22, 24.

The FIG. 3 arrangement may be modified and adapted as discussed above with reference to FIGS. 1 and 2. 

1. A vacuum pumping system for evacuating a vacuum chamber, the system comprising: a vacuum pump; and a plurality of forelines for conveying gas to the vacuum pump wherein in a first low vacuum stage of chamber evacuation a first foreline arrangement can be connected for conveying gas to the vacuum pump and in a second higher vacuum stage of chamber evacuation a second foreline arrangement comprising one or more of said forelines can be connected for conveying gas to the vacuum pump, wherein the second foreline arrangement has a total cross-sectional area for conveying fluid which is larger than a total cross-sectional area of the first foreline arrangement.
 2. A vacuum pumping system as claimed in claim 1, wherein at least one of said forelines in said second foreline arrangement is isolated from the first foreline arrangement during said relatively low vacuum stage of chamber evacuation.
 3. A vacuum pumping system as claimed in claim 1 or 2, wherein said second foreline arrangement comprises one or more of said forelines of said first foreline arrangement.
 4. A vacuum pumping system as claimed in claim 3, wherein said second foreline arrangement comprises said plurality of forelines.
 5. A vacuum pumping system as claimed in any one of the preceding claims, wherein said first foreline arrangement comprises a first foreline and said second foreline arrangement comprises said first foreline and a second foreline.
 6. A vacuum pumping system as claimed in claim 5, wherein said first foreline has a first cross-sectional area which is smaller than the cross-sectional area of the second foreline.
 7. A vacuum pumping system as claimed in claim 6, wherein said first foreline has a cross-sectional area in the range of 10⁴ to 10⁵ mm² and said second foreline has a cross-sectional area in the range of 10⁵ to 10⁶ mm².
 8. A vacuum pumping system as claimed in any one of the preceding claims, comprising a control unit configured for selecting said first foreline arrangement for conveying gas to said vacuum pump during said first relatively low vacuum stage of chamber evacuation and selecting said second foreline arrangement for conveying gas to said vacuum pump when said pressure is reduced below a predetermined pressure or at a predetermined time from commencement of chamber of evacuation.
 9. A vacuum pumping system as claimed in claim 8, wherein said predetermined pressure is in the range of 0.1 mbar to 10 mbar.
 10. A vacuum pumping system as claimed in claim 8 or 9, wherein said control unit is operably connected to at least one valve for controlling the flow of gas through the first foreline arrangement or the second foreline arrangement according to a control signal received from the control unit.
 11. A vacuum pumping system as claimed in claim 10, comprising a first valve upstream of the second foreline and a second valve downstream of the second foreline for isolating the second foreline from the vacuum pumping system.
 12. A vacuum pumping system as claimed in any one of claims 8 to 11, wherein said control unit is operably connected to receive a pressure signal from a pressure sensor for sensing a pressure in the vacuum pumping system, wherein said control unit is configured to output a control signal when said pressure signal indicates a reduction of pressure in said system below a predetermined amount.
 13. A vacuum pumping system as claimed in any preceding claim, wherein said first foreline arrangement has a volume which is at least an order of magnitude less than the volume of the vacuum chamber and the second foreline arrangement has a volume which is of the same order of magnitude or greater than the volume of the vacuum chamber.
 14. A vacuum pumping system as claimed in any one of the preceding claims, wherein the total cross-sectional area of the second foreline arrangement is sized so that speed of evacuation is limited by a pumping speed of the vacuum pump and not by the conductance of the second foreline arrangement.
 15. A vacuum pumping system as claimed in any one of the preceding claims, wherein the vacuum pump is connected by one or both foreline arrangements are connected to a second vacuum pump which is connected to the vacuum chamber.
 16. A method of evacuating a vacuum chamber, the method comprising: connecting a first foreline arrangement for conveying fluid to a vacuum pump and evacuating gas through the first foreline arrangement in a first relatively low vacuum stage of chamber evacuation; and connecting a second foreline arrangement for conveying gas to the vacuum pump and evacuating gas through the second foreline arrangement in a second higher vacuum stage of chamber evacuation, wherein the second foreline arrangement has a total cross-sectional area which is larger than the total cross-sectional area of the first foreline arrangement. 